A contemplated application of the present invention is in an ion implanter that may be used in the manufacture of semiconductor devices or other materials, although many other applications are possible. In such an application, semiconductor wafers are modified by implanting atoms of desired dopant species into the body of the wafer to form regions of varying conductivity. Examples of common dopants are boron, phosphorus, arsenic and antimony.
Typically, an ion implanter contains an ion source held under vacuum within a vacuum chamber. The ion source produces ions using a plasma generated within an arc chamber. Plasma ions are extracted from the arc chamber and, in an “ion shower” mode, the ions travel to implant in a target such as a semiconductor wafer. Alternatively, the extracted ions may be passed through a mass analysis stage such that ions of a desired mass and energy are selected to travel onward to implant in a semiconductor wafer. A more detailed description of an ion implanter can be found in U.S. Pat. No. 4,754,200.
In a typical Bernas-type source, thermal electrons are emitted and accelerated under the influence of an electric field from a cathode and are constrained by a magnetic field to travel along spiral paths towards a counter-cathode. Interactions with precursor gas molecules within the arc chamber produces the desired plasma.
In one known arrangement, the counter-cathode is connected to the cathode such that they are at a common potential (U.S. Pat. Nos. 5,517,077 and 5,977,552). The counter-cathode is negatively-biased so that it repels electrons travelling from the cathode, increasing the number of spiral paths across the ion source thereby increasing ionisation efficiency in the arc chamber.
In another known arrangement, the counter-cathode is electrically isolated so that it floats to close to the potential of the plasma (U.S. Pat. No. 5,703,372).
The mass analysis stage of the implanter is operated by control of a magnetic field to allow selection of ions of a desired mass (via their momentum or mass to charge-state ratio) and rejection of unwanted ions (to the extent that they follow a different path in the magnetic field).
In the case of boron doping, for example, BF3 is normally used as a precursor gas. Dissociation in the arc chamber results in a plasma typically containing B+, F+, BF+ and BF2+ ions. This mixture of ions is extracted and enters the mass analysis stage which ensures that only the preferred B/BFx species is delivered to the semiconductor wafer. Although many implant recipes require B+ ions to be implanted, others use BF2+ ions. Because the BF2+ ions dissociate on impact with a semiconductor wafer, the resulting boron atoms are implanted with reduced energy yielding shallower doping layers as is required in some applications.